But, other people involved (Mike Gorse, Joanie) can confirm/complete
all this.

I'll confirm: What your wrote is a good summary of the situation. Thanks!

In terms of "complete all this": GS-mag supports another route for
controlling its settings, namely, GConf. Furthermore, that part of the
code is in the process of being converted to GSettings [1]. The idea is
that by changing a user's session wide magnification preferences via
GSettings live-updates the magnifier's settings. A concrete example is
setting the mouse tracking preference in Orca's magnifier when running
gs-mag. In this case, Orca uses GConf to change the user's magnifier
mouse tracking desktop preferences. GS-mag "hears" that change and
responds appropriately.

A couple of related issues/tasks are that Orca's preferences [2] , and
desktop a11y preferences [3] are also migrating to the GSettings solution.